Duodenal cells from vitamin D-deficient rats will be incubated with 1,25-dihydroxy-vitamin D3 in the presence or absence of Ca2 ion to determine whether calcium inhibits the induction of the calcium-binding protein. Concentration-dependent calcium uptake of induced and uninduced cells will be compared to determine whether induced cells behave like cells from vitamin D-replete animals. Brush border vesicles will be prepared from these cells to determine whether CaBP affects the entry component of the 3 step transport process: entry, transcellular movement, and energy-dependent extrusion.